The invention relates to the identification and sorting of plastic refuse. Specifically, the invention involves the application of Raman spectroscopy to identification and sorting of post-consumer plastics for recycling.
Society has reaped much benefit from the use of plastics over the past decades. The strength, lightness and versatility of products made from plastic makes their use advantageous over many other materials. Disposal of the products made from plastics, however, has clearly been a problem. Post-consumer plastics have become a tremendous burden upon our waste disposal systems. Plastics constitute only about 9% by weight of municipal solid waste but they occupy approximately 25% of the volume of the waste stream.
Recycling is one major solution to the problem of disposal. Diversion of plastics from landfill to recycling can potentially save the energy equivalent of 60 million barrels of oil annually. In addition, new applications for and products made from recycled post-consumer plastics are being developed each day. Some of these applications and product include the production of fiber for clothing and carpeting, durable goods, consumer goods packaging, and food and beverage packaging.
However, post-consumer plastics recycling is expensive and not cost-effective when the plastics are unsorted. Mixed plastics are of little or no economic value and have limited marketability. To be cost-effective, it is necessary for mixed plastics to be sorted according to chemical composition.
Currently, there are various techniques for identifying and sorting materials by polymer type. Some of these techniques include manual hand sorting, density separation, and various automated "sense/sort" systems. Hand sorting is tedious, expensive, prone to error, and can be unsafe. Density separation of granulated plastics by sink/float in a water bath yields only a float product and a sink product and is not useful for a primary sort of mixed plastics. Sensing methods used in automated sense/sort systems include x-ray analysis, optical inspection using photodiodes or CCD machine vision, and near infrared (NIR) sensing. X-ray analysis is effective only for separating polyvinyl chloride (PVC) plastics from polyethylene terephthalate (PETE) plastics. Optical scanning of post consumer plastics is useful for sorting plastics according to transparency and color but is unable to provide chemical identification of polymers. Application of NIR technology to plastic waste sorting remains largely unproven. A new cost effective technology is needed that can accurately identify and sort plastics by polymer type at high throughput rates.